Previoulsy this has happened and i changed the DHW sensor and instaled new temp sensors in the heating elements.

This time i have changed the DHW sensor again, the heating elements are reading 19 homs +-.5 i believe this is ok?. but still there not heating up, normally the brass threads are tepid to the touch! neither the top or bottom are at the moment.. any ideas please??

also can you confirm that when the off peak is on only the bottom element is supposed to be heating and drawing current and when i turn on the boost then the top element starts to hea up and the biottom one doesnt?